The Leggett–Garg inequality attempts to classify experimental outcomes as arising from one of two possible classes of physical theories: those described by macrorealism (which obey our intuition about how the macroscopic classical world behaves) and those that are not (e.g., quantum theory). The development of cloud-based quantum computing devices enables us to explore the limits of macrorealism. In particular, here we take advantage of the properties of the programmable nature of the IBM quantum experience to observe the violation of the Leggett–Garg inequality (in the form of a ‘quantum witness’) as a function of the number of constituent systems (qubits), while simultaneously maximizing the ‘disconnectivity’, a potential measure of macroscopicity, between constituents. Our results show that two- and four-qubit ‘cat states’ (which have large disconnectivity) are seen to violate the inequality, and hence can be classified as non-macrorealistic. In contrast, a six-qubit cat state does not violate the ‘quantum witness’ beyond a so-called clumsy invasive-measurement bound, and thus is compatible with ‘clumsy macrorealism’. As a comparison, we also consider un-entangled product states with n = 2, 3, 4 and 6 qubits, in which the disconnectivity is low.

Leggett-Garg不等式试图将实验结果归类为两种可能的物理理论之一：宏观现实主义（遵循我们对宏观古典世界的行为的直觉）和非现实主义（例如，量子理论） 。基于云的量子计算设备的发展使我们能够探索宏观现实的极限。特别是，在这里，我们利用IBM量子经验的可编程性质，来观察违反Leggett–Garg不等式（以“量子见证”的形式）随组成系统数量的变化（ qubits），同时最大程度地提高了成分之间的“不连通性”（宏观性的一种潜在度量）。我们的结果表明，两个和四个量子位的“猫状态”（具有很大的不连续性）被视为违反了不等式，因此可以归类为非宏观现实。相比之下，六比特的猫状态在所谓的笨拙的入侵测量范围内并没有违反“量子见证”，因此与“笨拙的宏观现实”兼容。作为比较，我们还考虑了无纠缠的产品状态n  = 2、3、4和6个量子位，其中不连通性很低。